1. Field of the Invention
The present invention relates to a hook assembly which is incorporated into a lockstitch sewing machine or the like as well as to a sewing process utilizing such hook assembly.
2. Description of the Related Art
A bobbin case basket assembly which is incorporated into a lockstitch sewing machine has a bearing rib formed on its outside circumference, which bearing rib is fitted into a groove shaped bearing raceway formed in the inside circumference of a hook. The hook is rotated at a high speed while the bobbin case basket assembly is prevented from rotating downstream in the direction of rotation of the hook by a position finger secured to the body of the sewing machine. Specifically, hooks of industrial sewing machines are rotated at high speeds of about 6,000-10,000 rpm.
Accordingly, a large frictional force is generated due to the sliding contact between the bearing rib and the groove shaped bearing raceway. In cases where the friction between the bearing rib and the groove shaped bearing raceway is high, the position finger is brought into vigorous contact with the side wall of a rotation restraining groove of the bobbin case basket assembly. Therefore a needle thread cannot pass smoothly through the rotation restraining groove. This causes the tension of the needle thread to change undesirably and the sewing quality to worsen. Durability is also diminished due to the friction between the bearing rib and the groove shaped bearing raceway.
In order to solve this problem, there is a well known construction designed to provide a lubricant between the surface layer, or the outside circumference, of the bearing rib on the bobbin case basket assembly and the inside circumference of the groove shaped bearing raceway in the hook. This construction, however, has the disadvantage that the needle thread, bobbin thread, cloth, etc., become stained by the lubricant. Accordingly, another conventional construction includes coating of the bearing rib on the bobbin case basket assembly with a fluorine resin with a low friction coefficient, such as TEFLON (trade name). According to the techniques of such prior art arrangement, however, since the fluorine resin is sprayed and baked to provide a thin film coating usually around 10-20 .mu.m thick, the durability thereof is poor, and particularly in cases where no oil is fed, there is an additional problem that such coating readily peels off.